The invention relates generally to integrated circuit fabrication and more specifically to metrology for process control during semiconductor wafer manufacturing.
During semiconductor fabrication there are many opportunities for measuring features of substrates undergoing processing operations. Many of the features can be determined by capturing a signal indicating the feature. For example, various end point determination methods are available that employ laser interferometry, optical emission, etc. However, as the features continue to decrease in size, especially the thickness of films employed in the manufacture of semiconductors, the signals that are indicative of the feature become undetectable in certain situations. For example, eddy current sensors are used for displacement, proximity and film thickness measurements. The sensors rely on the induction of current in a sample by the fluctuating electromagnetic field of a test coil proximate to the object being measured. Fluctuating electromagnetic fields are created as a result of passing an alternating current through the coil. The fluctuating electromagnetic fields induce eddy currents which perturb the applied field and change the coils inductance.
FIG. 1 is a simplified schematic diagram of the principle upon which an eddy current sensor operates. An alternating current flows through coil 100 in close proximity to conducting object 102. The electromagnetic field of the coil induces eddy currents 104 in conducting object 102. The magnitude and the phase of the eddy currents in turn effect the loading on the coil. Thus, the impedance of the coil is impacted by the eddy currents. This impact is measured to sense the proximity of conducting object 102 as well as a thickness of the object. Distance 106 impacts the effect of eddy currents 104 on coil 100, therefore, if object 1002 moves, the signal from the sensor monitoring the impact of eddy currents on coil 100 will also change.
Attempts to use eddy current sensors to measure thickness of a thin film, especially a copper thin film, have been unsuccessful. The eddy current sensors have been found to be blind to films having a thickness less than about 2500 Angstroms (Å). Accordingly, as feature sizes continue to decrease, eddy current sensors must be able to be adapted to sense thin films less than about 2500 Å. In addition, thickness measurement sensors currently available are generally designed to measure the thickness of a particular layer of a layer stack. That is, the sensors are specifically designed to measure thickness solely and do not have the flexibility to perform other measurements besides thickness measurements. More particularly, the thickness measurement sensors do not have the capability of selectively enhancing a thickness measurement for a particular layer of a stack of layers disposed over a substrate.
In view of the foregoing, there is a need to enhance the sensitivity of eddy current sensors to allow for the measurement of thin films. Additionally, there is a need to provide a sensor capable of selectively targeting a film of a layer stack.